A problem exists with respect to digital clocks as used in automobiles which must have their display output brightness selectively varied in accordance with ambient light conditions. This adjustability must extend over a range of voltage greater than that normally used in the standard dashboard or headlight brightness adjustment circuits. It is further important that this adjustment of voltage be substantially linear throughout the adjustment range. The present invention will be seen to achieve these objectives and offer these advantages even though the circuit is a simple one employing a single transistor and two-terminal elements such that the circuit is economical to build and use. The circuit, it will be appreciated, has a large number of applications in addition to the one shown in the preferred embodiment.